1) Field of the Invention
The invention relates to measurement of a wood fiber suspension, and particularly to optical measurement of kappa number and brightness.
2) Description of Related Art
The purpose of paper and pulp measurements is to ensure an end product of good quality. In the pulp industry, measurement of the pulp lignin content is one of the most important basic variables of pulp quality. The lignin content can be measured in the laboratory gravimetrically by hydrolyzing carbon hydrates with an acid, or using a xe2x80x98kappa numberxe2x80x99. In the standard the kappa number is defined as the amount of potassium permanganate solution with a concentration of 20 mmol/l in milliliters which one gram of dry pulp consumes in the conditions defined in the standard. This measurement is defined in greater detail in standard SCAN-C 1:77, which is incorporated herein by reference. The lignin content is about 0.15 to 0.2 times the kappa number, depending on the pulp. In the manufacture of pulp it is important to obtain real-time information on the different process stages, which enables quick control of the process. So fast measurement and control of the process cannot, however, be implemented by means of the laboratory measurement of kappa number.
Instead of laboratory measurements, it is nowadays common to use optical on-line kappa analyzers which measure lignin by means of ultraviolet radiation. In general, the measurement is based on the Lambert-Beer Law, i.e. the measurement is performed by measuring absorbance A of the suspension by means of consistency c, the distance L traveled by radiation in the suspension and an absorption constant. Mathematically the absorbance A can be expressed as follows A=c*L*xcex1. In other words, the measurement determines absorption of ultraviolet radiation from a pulp with a desired consistency. One particular problem related to such optical measurement is that the accuracy of the lignin content measurement depends on the relative number of fibers, which is expressed as the consistency c. It is difficult to set the consistency to a certain value as well as to measure it accurately because the consistency fluctuates in the fiber suspension measurement. This leads to an inaccurate measurement result of the kappa number.
The brightness of pulp or paper is often measured by means of the ISO brightness. This measurement is explained more closely in standard SCAN-P 3:93, which is incorporated herein by reference. In the laboratory measurement of brightness defined in the standard the brightness is determined from a dry pulp sheet. The measurement of pulp brightness is also based on the Lambert-Beer Law in the same way as the optical measurement of the kappa number, except for that the paper or pulp brightness is measured with optical radiation having a wavelength of 457 nm. The problems are similar to those related to the kappa number measurement. The measurement accuracy of brightness depends on the measurement accuracy of consistency, and since it is difficult to set the consistency to a certain value, the measurement result of brightness is inaccurate. The information provided by the brightness signal also has a certain optimum, which depends on the consistency.
An object of the invention is to provide an improved method and an apparatus implementing the method to reduce or avoid the above-mentioned problems. This is achieved with a method of measuring a suspension which contains wood fibers, the method comprising directing optical radiation at pulp and measuring optical radiation emitted by the pulp. The method is characterized by measuring at least one of the following two properties: kappa number and brightness; and the method comprises changing the pulp consistency in a desired range of consistency extending from a desired initial consistency to a desired final consistency; measuring the pulp consistency; measuring the strength of a desired wave length from the optical radiation at different consistencies in the desired consistency range; modeling measurement points with a function of the desired type by fitting the parameters of the function according to the measurement points; and determining at least one of the following two properties of the pulp by means of the modeling function: kappa number and brightness.
The invention also relates to a measurement device for measuring a suspension which contains wood fibers, the measurement device comprising an optical power source for directing optical radiation at the suspension and at least one detector for measuring optical radiation emitted by the suspension. The measurement device according to the invention is characterized in that the measurement device is arranged to measure at least one of the following two properties: kappa number and brightness; and the measurement device is arranged to change the pulp consistency in a desired range of consistency extending from a desired initial consistency to a desired final consistency; and the measurement device is arranged to measure the pulp consistency; measure the strength of a desired wave length from the optical radiation at different consistencies in the desired consistency range; model measurement points with a function of the desired type by fitting the parameters of the function according to the measurement points; and determine at least one of the following two properties of the pulp by means of the modeling function: kappa number and brightness.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on changing the suspension consistency and on continuous and simultaneous measurement of the strength of optical radiation, which provides a set of measurement points for the strength of optical radiation in the desired consistency range. A continuous function is fitted to the set of measurement points, which provides continuous dependency between the consistency and the strength of optical radiation in the desired consistency range. Finally, the desired pulp property, such as kappa number and brightness, is determined from the behavior of the continuous function.
The method and arrangement according to the invention provide several advantages. The measurement is fast because the consistency is not set to a certain value for the measurement, but optical measurements are performed by sliding the consistency. This improves the reproducibility and accuracy of the measurement. The fact that the measurement is performed from flowing pulp also improves it.